Characterization of fungicide resistance in Cercospora beticola populations from sugarbeet (Beta vulgaris) in Michigan

Authors: HERNANDEZ, ALEXANDRA - Michigan State University; RUTH, SARAH - Michigan State University; SATO-CRUZ, MIO - Michigan State University; BUBLITZ, DANIEL - Michigan Department Of Agriculture; Hanson, Linda; WILLBUR, JAIME - Michigan State University

Submitted to: PhytoFrontiers
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/18/2025
Publication Date: 11/13/2025
Citation: Hernandez, A.P., Ruth, S., Sato-Cruz, M., Bublitz, D.M., Hanson, L.E., Willbur, J.F. 2025. Characterization of fungicide resistance in Cercospora beticola populations from sugarbeet (Beta vulgaris) in Michigan. PhytoFrontiers. 5:651-662. https://doi.org/10.1094/PHYTOFR-03-25-0028-R.
DOI: https://doi.org/10.1094/PHYTOFR-03-25-0028-R

Interpretive Summary: Sugarbeet accounts for over half of domestic sugar production in the US, and Michigan is the fourth state for production. However, disease impacts this production. Specifically, Cercospora leaf spot (CLS) is one of the most damaging diseases in the area, and around much of the world. Management of the causal agent, Cercospora beticola, relies heavily on use of fungicides. There are applied repeatedly in a year for disease management, and fungicide resistance has been found for several of the important fungicide groups. In 2021 and 2022, fungal isolates were screened for their response to eight different fungicides belonging to four different fungicide classes. There were one each in the organo-tin, methylbenzimidazole carbamate (MBC) and quinone outside inhibitor (QoI) classes, and five in from the demethylation inhibitor (DMI) class. For the DMI fungicides, resistance to one member of the class was usually related to resistance to other members of the class, although there were some exceptions, with up to 85% correlation between responses. For the MBC and two of the DMI fungicide, the amount of resistance observed showed a relationship to the number of fungicide treatments, but this was not true for all fungicides tested. Genetic changes that have been associated with DMI, MBC, and QoI resistances were tested for use in rapidly screening strains. While the MBC resistance behavior was strongly related to presence of the genetic change, the other two fungicide classes did not show such a strong relationship. During the study, reduced sensitivity was observed to all tested fungicides, however, the frequency varied, with resistance to the MBC (71-85%), two DMI fungicides (79-96%) and the QoI (98-100%) the most common. This information is important for making management decisions. Finding more reliable genetic markers could speed up detection, allowing for more rapid management recommendations.

Technical Abstract: Sugarbeets produce over half of domestic sugar, and Michigan is the fourth highest U.S. producer. Cercospora beticola causes one of the most impactful foliar diseases in sugarbeet growing regions, Cercospora leaf spot (CLS). Management of CLS relies heavily on timely and repeated fungicide applications, and fungicide resistance has been observed for several fungicide groups in C. beticola. In 2021 and 2022, in vitro fungicide sensitivity was tested for five demethylation inhibitors (DMI; difenoconazole, fenbuconazole, mefentrifluconazole, prothioconazole, tetraconazole), a quinone outside inhibitor (QoI; pyraclostrobin), a methyl benzimidazole carbamate (MBC; thiophanate-methyl), and triphenyltin hydroxide. Cross resistance was detected between most DMIs with strongest correlations between difenoconazole and mefentrifluconazole (r = 0.85, P < 0.0001) and prothioconazole and tetraconazole (r = 0.61, P < 0.0001). Fungicide application data indicated that C. beticola sensitivity to thiophanate-methyl, tetraconazole, and prothioconazole was affected by the number of MBC and DMI applications, respectively (P < 0.05). Mutations associated with resistance to QoI, MBC, and DMI fungicides also were evaluated using restriction fragment length polymorphisms (PCR-RFLP). While the E198A mutation was significantly associated with C. beticola responses to thiophanate-methyl (P < 0.0001), G143A and Glu169 were not strongly associated with pyraclostrobin or prothioconazole and tetraconazole responses, respectively. Overall, in vitro insensitivity (EC50 > 1 µg ml-1) was observed for all tested active ingredients, however, frequencies were greatest for thiophanate-methyl (71-85%), tetraconazole (79-95%), prothioconazole (96%), and pyraclostrobin (98-100%). Identifying mutations more explanatory of resistance to DMI and QoI fungicides in C. beticola populations would improve timely monitoring efforts for this issue.